It actually is a NOAA/NWS data map that I imported into a mapping software program. They use NWS "Normals" from 1980 - 2010.

According to NWS Memphis - the average January high is 49.8 ....so they are just below 50 F. Not sure about the smaller cities near the AL/MS line, but I don't think any NWS station in southern TN has a average January high of 50 F or above.